Compact multi-resolution aerial camera system

ABSTRACT

A system for capturing aerial images, the system comprising at least one overview camera, a plurality of detail cameras, and a frame for holding the cameras, each detail camera having a longer focal length than the at least one overview camera, each detail camera mounted at a different angle laterally so that the fields of view of the detail cameras overlap to form an extended lateral field of view, the frame attachable to the floor of an aircraft above a camera hole, thereby providing the cameras with a view of the ground below the aircraft through the camera hole.

FIELD OF THE INVENTION

The present invention relates to aerial camera systems and efficientmethods for creating photomosaics from aerial photos.

BACKGROUND OF THE INVENTION

Accurately georeferenced photomosaics of orthophotos are becomingpopular alternatives to traditional pictorial maps because they can becreated automatically from aerial photos, and because they show actualuseful detail on the ground.

The creation of accurate photomosaics from aerial photos is welldescribed in the literature. See, for example, Elements ofPhotogrammetry with Application in GIS, Fourth Edition (Wolf et al.),and the Manual of Photogrammetry, Sixth Edition (American Society forPhotogrammetry and Remote Sensing (ASPRS)).

The creation of a photomosaic requires the systematic capture ofoverlapping aerial photos of the area of interest, both to ensurecomplete coverage of the area of interest, and to ensure that there issufficient redundancy in the imagery to allow accurate bundleadjustment, orthorectification and alignment of the photos.

Bundle adjustment is the process by which redundant estimates of groundpoints and camera poses are refined. Modern bundle adjustment isdescribed in detail in “Bundle Adjustment—A Modern Synthesis” (Triggs etal.).

Bundle adjustment may operate on the positions of manually-identifiedground points, or, increasingly, on the positions ofautomatically-identified ground features which are automatically matchedbetween overlapping photos.

Overlapping aerial photos are typically captured by navigating a surveyaircraft in a serpentine pattern over the area of interest. The surveyaircraft carries an aerial camera system, and the serpentine flightpattern ensures that the photos captured by the camera system overlapboth along flight lines within the flight pattern and between adjacentflight lines.

Sufficient redundancy for accurate bundle adjustment typically dictatesthe choice a longitudinal (forward) overlap of at least 60%, i.e.between successive photos along a flight line, and a lateral (side)overlap of at least 40%, i.e. between photos on adjacent flight lines.This is often referred to as 60/40 overlap.

The chosen overlap determines both the required flying time and thenumber of photos captured (and subsequently processed). High overlap istherefore expensive, both in terms of flying time and processing time,and practical choices of overlap represent a compromise between cost andphotomosaic accuracy.

The use of a multi-resolution camera system provides a powerful way toreduce overlap without excessively compromising accuracy. The captureand processing of multi-resolution aerial photos is described in U.S.Pat. Nos. 8,497,905 and 8,675,068 (Nixon), the contents of which areherein incorporated by cross-reference. Multi-resolution sets of photosallow photomosaic accuracy to be derived from the overlap betweenlower-resolution overview photos, while photomosaic detail is derivedfrom higher-resolution detail photos.

U.S. Pat. Nos. 8,497,905 and 8,675,068 (Nixon) describe an externalcamera pod attachable to a small aircraft. An external pod has two keydisadvantages: the pod is highly aircraft-specific, and space within thepod is constrained. An aircraft-specific pod limits the choice ofaircraft and therefore limits operational parameters such as altituderange, and, conversely, requires significant design, testing andcertification effort to adapt to different aircraft. Constrained spacewithin the pod limits the size and therefore the focal length of cameralenses, which in turn limits the range of operating altitudes for aparticular target image resolution.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a system for capturingaerial images, the system comprising at least one overview camera, aplurality of detail cameras, and a frame for holding the cameras, eachdetail camera having a longer focal length than the at least oneoverview camera, each detail camera mounted at a different anglelaterally so that the fields of view of the detail cameras overlap toform an extended lateral field of view, the frame attachable to thefloor of an aircraft above a camera hole, thereby providing the cameraswith a view of the ground below the aircraft through the camera hole.

The system may comprise an adapter plate attachable to the floor of theaircraft, the frame attachable to the adapter plate. For example, theadapter plate may attach to the floor by bolting to mounting points setinto the floor, or it may attach to the floor by bolting to seat tracksattached to the floor.

Each detail camera may be angled inwards towards the center of thecamera hole, thereby minimising the size of the camera hole required toaccommodate the fields of view of the detail cameras.

The ratios of the focal lengths of the detail cameras to the focallength of the at least one overview camera may be between 4 and 8.

The focal lengths of the detail cameras may be between 85 mm and 800 mm.Each detail camera may utilise a stock lens with a focal length such as85 mm, 105 mm, 135 mm, 180 mm, 200 mm, 300 mm, 400 mm, 500 mm, 600 mm,700 mm, or 800 mm.

The focal length of the overview camera may be between 10 mm and 200 mm.The overview camera may utilise a stock lens with a focal length such as10.5 mm, 14 mm, 16 mm, 18 mm, 20 mm, 21 mm, 24 mm, 28 mm, 35 mm, 40 mm,45 mm, 50 mm, 55 mm, or 60 mm.

The system may comprise five detail cameras.

The detail cameras may have fixed-focus lenses focused at infinity, orvariable-focus lenses and auto-focus mechanisms.

The overview camera has a fixed-focus lens focused at infinity, or avariable-focus lens and an auto-focus mechanism.

The system may comprise a computer system configured to automaticallyfire the detail cameras during flight such that successive detail photosoverlap longitudinally.

The system may comprise at least one Global Navigation Satellite System(GNSS) receiver, the computer configured to receive and store positiondata from the at least one GNSS receiver in real time.

The system may comprise an IMU, the computer configured to receive andstore orientation data from the IMU in real time.

The system may comprise a pilot display, the computer configured toprovide flight instructions to the pilot via the cockpit display.

The system may comprise a stored flight plan, the computer configured tofire the cameras at a rate determined from the flight plan, thereal-time position of the aircraft, and the real-time speed of theaircraft.

DRAWINGS—FIGURES

FIG. 1 shows a front view of the HyperCamera camera unit, i.e. facingforwards towards the front of the aircraft.

FIG. 2 shows a back view of the camera unit.

FIG. 3 shows a top view of the camera unit.

FIG. 4 shows a bottom view of the camera unit.

FIG. 5 shows an exploded view of the camera unit.

FIG. 6 shows an exploded view of the cameras and the central support ofthe camera unit, with the field of view of each camera.

FIG. 7 shows the camera unit from below, with its combined fields ofview passing through the aperture of an aircraft camera hole.

FIG. 8 shows the overview field of view and the five overlapping detailfields of view of the camera unit.

FIG. 9 shows the adapter plate of the HyperCamera mounted on seat trackson the floor of an aircraft.

FIG. 10 shows an exploded view of the camera unit, the adapter plate,and seat tracks on the floor of an aircraft.

FIG. 11 shows an exploded view of the cameras and the central support ofa wider-angle version of the camera unit.

FIG. 12 shows the overview field of view and the five overlapping detailfields of view of the wider-angle version of the camera unit.

FIG. 13 shows a plan view of the HyperCamera installed in a Cessna 208aircraft.

FIG. 14 shows a detailed plan view of the HyperCamera installed in aCessna 208 aircraft.

FIG. 15 shows a front elevation of a Cessna 208 aircraft carrying aHyperCamera, and the resultant overview and aggregate detail fields ofview.

FIG. 16 shows a side elevation of a Cessna 208 aircraft carrying aHyperCamera, and the resultant overview and aggregate detail fields ofview.

FIG. 17 shows the overlapping fields of view of three successive shots.

FIG. 18 shows the overlapping fields of view of shots in adjacent flightlines.

FIG. 19 shows the overlapping aggregate detail fields of view of asuccession of shots along three adjacent flight lines.

FIG. 20 shows a block diagram of a power and control system for theHyperCamera.

FIG. 21 shows a photogrammetric process flow for efficiently creating aphotomosaic from multi-resolution HyperCamera photos.

DRAWINGS—REFERENCE NUMERALS

-   -   100 Camera unit.    -   110 Detail camera.    -   112 Overview camera.    -   114 Detail camera lens.    -   116 Overview camera lens.    -   120 Frame.    -   122 Frame center support.    -   124 Frame side support.    -   126 Frame rear support.    -   128 Frame front support.    -   130 Mount point block.    -   132 Mount point.    -   134 Mount bolt.    -   140 Mount for detail camera.    -   142 Mount for overview camera.    -   144 Clamp for detail camera lens.    -   146 Clamp for overview camera lens.    -   150 Power and control distribution box.    -   160 Detail field of view.    -   162 Lateral detail field of view.    -   164 Longitudinal detail field of view.    -   170 Overview field of view.    -   172 Lateral overview field of view.    -   174 Longitudinal overview field of view.    -   180 Aggregate detail field of view.    -   182 Lateral aggregate detail field of view.    -   200 Adapter plate.    -   202 Seat track fastener.    -   210 Aircraft floor.    -   212 Camera hole.    -   214 Seat track.    -   216 Adapter plate aperture.    -   220 Direction of flight.    -   222 Flight path.    -   224 Shot position.    -   230 Aerial survey aircraft.    -   300 Computer.    -   302 Pilot display.    -   304 Inertial Measurement Unit (IMU).    -   306 Global Navigation Satellite System (GNSS) receiver.    -   308 Analog-to-digital converters (ADCs).    -   310 Camera control unit (CCU).    -   320 Battery unit.    -   322 Aircraft auxiliary power.    -   324 Ground power unit (GPU).    -   400 Detail photos.    -   402 Overview photos.    -   404 Photomosaic.    -   410 Match features step.    -   412 Solve pose and positions step.    -   414 Orthorectify step.    -   416 Blend step.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The HyperCamera™ is a compact multi-resolution aerial camera systemsuitable for easy deployment in a wide range of aircraft, large andsmall. It is designed to be installed above a standard 20-inch camerahole, as is often provided through the floor of a survey aircraft orairborne pod.

In a preferred embodiment, as illustrated in FIGS. 1 through 5, theHyperCamera comprises a camera unit 100 incorporating five detailcameras 110 and a relatively wider-angle overview camera 112. Eachdetail camera 110 has a detail lens 114, and the overview camera 112 hasa overview lens 116.

The overview lens 116 is characterised by having a significantly widerangle than the detail lens 114. While it may be a true wide-angle lens,it may also be a normal lens or even a telephoto lens so long as it issignificantly wider than the detail lens 114. Likewise, while the detaillens 114 may be a true telephoto lens, it may also be a normal lens oreven a wide-angle lens so long as it is significantly narrower than theoverview lens 116.

The cameras 110 and 112 are commercial off-the-shelf (COTS) digital SLR(DSLR) cameras. The use of COTS cameras allows the system to be readilyadapted to the latest and best available cameras.

High-resolution COTS cameras are available with typical pixel countsranging from 24 Mpixels to 36 Mpixels, from vendors such as Nikon andCanon. The 36 Mpixel Nikon D800 DSLR camera is a particularly goodchoice for the present system.

DSLR cameras offer a wide range of high-quality lenses, allowing thesystem to be readily configured to operate at different altitudes andresolutions.

The system is readily adapted to a mixture of cameras. For example, arelatively more expensive camera with a higher pixel count may beemployed as the overview camera. 70 Mpixel DSLR cameras are expected tobe available in the near future, and a 70 Mpixel camera would be a goodchoice for the overview camera.

In the preferred embodiment the detail lenses 114 of the detail cameras110 all have the same focal length, and the detail cameras 110 all havethe same pixel size. Thus the camera unit 100 embodies two distinctcamera resolutions—overview and detail. This is readily extended tomultiple resolutions greater than two through the use detail lenses 114with different focal lengths, and/or the use of detail cameras 110 withdifferent pixel sizes. The camera unit 100 may also incorporate multipleoverview cameras with different resolutions.

Each detail lens 114 and overview lens 116 may be a fixed-focus lensfocused at infinity or a variable-focus lens. In the latter case thecorresponding camera 110 and/or 112 incorporates an auto-focusmechanism.

Each detail camera 110 is bolted to a camera mount 140, which in turn isbolted to a center support 122. Each detail camera lens 114 is furthersecured by a clamp 144 which is bolted to the detail camera mount 140.

The overview camera is bolted to a camera mount 142, which in turn isbolted to the center support 122. The overview camera lens 116 isfurther secured by a clamp 146 which is bolted to the overview cameramount 142.

The camera mounts 140 and 142 isolate much of the structure of cameraunit 100 from the specifics of individual camera models and lens sizes.

The center support 122 is attached to a pair of side supports 124 a and124 b, and each side support 124 is in turn attached to a rear support126 and a front support 128 to form a rigid frame 120.

Each side support 124 is attached to mount point block 130 via a set offour bolts, and the mount point block 130 is in turn attached to therear support 126 or front support 128, as appropriate, via a further setof four bolts. The mount point blocks 130 thereby provide the attachmentmechanism between the side supports 124 and the rear and front supports126 and 128.

Each of the side supports 124 and the rear and front supports 126 and128 has a C-shaped cross-sectional profile to minimise weight whilemaximising rigidity, while the center support 122 is pocketed tominimise weight while maximising rigidity.

Each mount point block 130 is solid, and serves the additional purposeof providing a point of attachment between the camera unit 100 and asurvey aircraft, as described below.

All parts are made from light-weight aluminium, except for fastenerswhich are made from steel.

The rear support 126 and the front support 128 hold three power andcontrol distribution boxes 150. Each box 150 distributes power andcontrol signals to a pair of cameras. For clarity, the power and controlcabling between the boxes 150 and the cameras 110 and 112 is omitted inthe figures.

In the preferred embodiment each detail camera 110 has a lens 114 with afocal length of 300 mm suitable for high-resolution imaging atrelatively high altitudes. For example, when using a 36 Mpixel NikonD800 camera (which has 4.88 um pixels), a 300 mm lens allows a groundsampling distance (GSD) of 10 cm at 20,000 feet, 8 cm at 16,000 feet, 6cm at 12,000 feet, 4 cm at 8,000 feet, 2 cm at 4,000 feet, and so on.

Assuming the detail cameras 110 and overview camera 112 have similarpixel counts and pixel sizes, the overview camera 112 ideally has a lens116 with a focal length that is between 4 and 8 times shorter than thefocal length of the detail lens 114, as discussed further below. I.e.for a 300 mm detail lens 114, suitable focal lengths for the overviewlens 116 range from about 40 mm to 75 mm. For illustrative purposes thepresent system utilises a 50 mm overview lens 116.

FIG. 6 shows the 6.90-degree lateral field of view 162 of each of thefive detail cameras 110 with 300 mm lenses 114, and the 39.60-degreelateral field of the overview camera 112 with a 50 mm lens 116.

In this specification, the lateral direction is the directionperpendicular to the direction of flight 220, and the longitudinaldirection is the direction parallel to the direction of flight 220.

As shown, the detail cameras are angled 6 degrees apart laterally, i.e.slightly less than their 6.90-degree fields of view 162, so that thefields of view 162 overlap slightly.

Using 36 Mpixel Nikon D800 cameras, the five detail cameras 110 have anaggregate field of view with a pixel count of approximately 160 Mpixels,i.e. excluding overlap.

Stock telephoto lenses suitable for use as detail lenses 114 areavailable in a variety of focal lengths, typically including 85 mm, 105mm, 135 mm, 180 mm, 200 mm, 300 mm, 400 mm, 500 mm, 600 mm, 700 mm, and800 mm.

At 20,000 feet a 400 mm lens on a Nikon D800 camera allows a GSD of 7.4cm, a 600 mm lens a GSD of 5.0 cm, and an 800 mm lens a GSD of 3.7 cm.

Stock normal and wide-angle lenses suitable for use as an overview lens116 are available in a variety of focal lengths, typically including10.5 mm, 14 mm, 16 mm, 18 mm, 20 mm, 21 mm, 24 mm, 28 mm, 35 mm, 40 mm,45 mm, 50 mm, 55 mm, 60 mm and 70 mm.

The camera unit 100 is readily adapted for different models and sizes ofcameras 110 (and 112) and lenses 114 (and 116) via different cameramounts 140 (and 142) and clamps 144 (and 146). For extremely long lensesa taller center support 122 can be used.

As shown in FIG. 6 and FIG. 7, the detail cameras are angled inwards sothat their fields of view 162 cross over directly below the camera unit100, creating a waist of minimum diameter where the fields of view passthrough the camera hole 212. This makes the camera unit 100 compatiblewith standard 20-inch camera holes, as well as camera holes as small asabout 17 inches.

FIG. 8 shows the projection of the three-dimensional fields of view 160and 170 of the detail cameras 110 and overview camera 112 onto a groundplane. It shows how the detail field of views 160 overlap in a directionperpendicular to the direction of flight 220.

FIG. 9 shows an adapter plate 200 that attaches to the seat tracks 214of an aircraft, a Cessna 208 in this case, via standard seat trackfasteners 202. The adapter plate has an aperture 216 which exposes acamera hole 212 through the floor 210 of the aircraft.

FIG. 10 shows an exploded view of the camera unit 100, adapter plate200, and the aircraft floor 210. The adapter plate 200 is designed toattach the camera unit 100 to a particular aircraft, and isolates thedesign of the camera unit 100 from aircraft specifics. A differentadapter plate is designed for each aircraft attachment variation, e.g.due to different seat track spacings, or because the aircraft's camerahole installation includes its own mounting points.

Four mount points 132 are bolted to the adapter plate, with each mountpoint 132 mating with a recess in the base of its corresponding mountpoint block 134. A mount bolt 143 securely attaches each mount pointblock 134 to its corresponding mount point 132, thus attaching thecamera unit 100 to the adapter plate 200.

The adapter plate 200 allows the camera unit 100 to be easily installedin and subsequently removed from an aircraft via installation andremoval of the four mount bolts 143. The adapter plate 200 is itselfgenerally easily installed in and removed from an aircraft, requiring nomodification to the aircraft (assuming a suitable camera hole is alreadyinstalled). The installation of external camera pod is generally a muchmore complicated operation.

FIG. 11 shows a variant of the camera unit 100 that utilises shorter 180mm lenses 114 for the detail cameras 110, and a matching 28 mm lens 116for the overview camera 112.

When using a 36 Mpixel Nikon D800 camera (which has 4.88 um pixels), a180 mm lens allows a ground sampling distance (GSD) of 9.9 cm at 12,000feet, 8.3 cm at 10,000 feet, 6.6 cm at 8,000 feet, 5 cm at 6,000 feet,3.3 cm at 4,000 feet, and so on.

FIG. 11 shows the 11.40-degree lateral field of view 162 of each of thefive detail cameras 110 with 180 mm lenses 114, and the 65.50-degreelateral field of the overview camera 112 with a 28 mm lens 116.

As shown, the detail cameras are angled 10.50 degrees apart laterally,i.e. slightly less than their 11.40-degree fields of view 162, so thatthe fields of view 162 overlap slightly.

FIG. 12 shows the projection of the three-dimensional fields of view 160and 170 of the detail cameras 110 and overview camera 112 of FIG. 10onto a ground plane. It shows how the detail field of views 160 overlapin a direction perpendicular to the direction of flight 220, and how thewider fields of view associated with the shorter lenses leads to a loweroperational altitude for the same footprint on the ground, i.e. incomparison to FIG. 8.

FIG. 13 and FIG. 14 show plan views of a Cessna 208 survey aircraft 230carrying a camera unit 100 installed centrally over a camera hole. Thefigures also show a camera control unit 310 (CCU) and battery unit 320used to control and power the camera unit 100. These are described inmore detail below. For clarity, the cabling connecting the CCU 310,battery unit 320 and camera unit 100 is omitted.

FIG. 15 shows a front elevation of the Cessna 208 survey aircraft 230carrying a HyperCamera, and shows the lateral overview field of view 172of the camera unit 100, and the aggregate lateral detail field of view182 of the camera unit 100. The aggregate lateral detail field of view182 is the aggregate of the five individual overlapping lateral detailfields of view 162.

FIG. 16 shows a side elevation of the Cessna 208 survey aircraft 230carrying a HyperCamera, and shows the longitudinal overview field ofview 174 of the camera unit 100, and the longitudinal detail field ofview 164 of the camera unit 100.

FIG. 17 shows the overlapping overview fields of view 170 and aggregatedetail fields of view 180 of three successive shots in the direction offlight 220. The aggregate detail field of view 180 is the aggregate ofthe five individual overlapping detail fields of view 160. At the camerafiring rate illustrated in the figure (i.e. as implied by thelongitudinal overlap), the aggregate detail fields of view 180 overlapby about 20% longitudinally, while the overview fields of view 170overlap by about 85% longitudinally.

FIG. 18 shows the overlapping overview fields of view 170 and aggregatedetail fields of view 180 of two shots from adjacent flight lines, i.e.flown in opposite directions 220. At the flight-line spacing illustratedin the figure, the aggregate detail fields of view 180 overlap bybetween 20% and 25% laterally, while the overview fields of view 170overlap by about 40% laterally.

Assuming the detail cameras 110 and the overview camera 112 have similarpixel counts and pixel sizes, the size of the lateral overview field ofview 172 and the size of the lateral aggregate detail field of view 182are similar when the ratio of the focal length of the detail camera lens114 to the focal length of the overview camera lens 116 is about 6, anduseful lens combinations can be chosen with focal length ratios betweenabout 4 and 8.

FIG. 19 shows the overlapping aggregate detail fields of view 180 of asuccession of shots along three adjacent flight lines that are part of atypical serpentine flight path 222, i.e. a subset of the flight linesthat would make up a typical large-area survey. For clarity thecorresponding overview fields of view 170 are omitted. The figure alsoshows the shot position 224 corresponding to each aggregate detail fieldof view 180, i.e. the position of the survey aircraft 230.

As already noted, traditional single-resolution aerial surveys aretypically operated with 60/40 overlap, i.e. 60% forward (orlongitudinal) overlap, and 40% side (or lateral) overlap. With themulti-resolution HyperCamera operated as shown in FIGS. 17 through 19,overview photos are captured with better than 85/40 overlap, and detailphotos are captured with only 20/20 overlap at best.

Compared with a traditional single-resolution aerial camera system and acomparable aggregate detail pixel count (e.g. 160 Mpixel), theHyperCamera is between 2 and 3 times more efficient, as detailed below,with respect to both reduced survey flying time and fewer photos toprocess. The HyperCamera also has a higher efficiency than many aerialcamera systems due to its high (detail) pixel count alone.

As an alternative to capturing both overview and detail photos, theHyperCamera can be used to capture detail photos only, with higheroverlap (e.g. 60/40 rather than 20/20), to allow the creation of aphotomosaic with higher spatial accuracy, but at greater capture andprocessing cost. In this case the overview camera 112 can be omitted.

To analyse the relative efficiency of a multi-resolution HyperCamera,assume a multi-resolution HyperCamera configuration with a lateraloverlap of X %, a longitudinal overlap of Y %, N detail cameras 110, andM overview cameras 112, and for comparison, a single-resolutionHyperCamera configuration with lateral overlap of A %, longitudinaloverlap of B %, N detail cameras, and no overview camera. Assuming X issmaller than A, the improvement in lateral efficiency, as reflected in agreater flight-line spacing and shorter flying time and fewer detailphotos captured, is given by (1−X)/(1−A). Likewise, assuming Y issmaller than B, the improvement in longitudinal efficiency, as reflectedin a greater shot spacing and shorter flying time and fewer detailphotos captured, is given by (1−Y)/(1−B). The overall improvement inefficiency is given by (1−X)(1−Y)/(1−A)(1−B). This needs to bediscounted by the overhead of capturing overview photos, i.e. multipliedby a factor of (N/(N+M)). For X/Y=20/20, A/B=60/40, N=5, and M=1, thenet efficiency improvement is 2.2.

The greater efficiency comes at the cost of performing somephotogrammetric calculations at the lower resolution of the overviewcamera 112 rather than at the higher resolution of the detail cameras110. However, this is at least partially compensated for by the greateroverlap between overview photos than in traditional practice.

FIG. 20 shows a block diagram of a power and control system for thecamera unit 100. The detail cameras 110 and overview camera 112 arecontrolled by a computer 300 via a set of analog-to-digital converters308 (ADCs).

The computer 300 uses one or more Global Navigation Satellite System(GNSS) receiver 304 to monitor the position and speed of the surveyaircraft 230 in real time. The GNSS receiver(s) may be compatible with avariety of space-based satellite navigation systems, including theGlobal Positioning System (GPS), GLONASS, Galileo and BeiDou.

The computer 300 provides precisely-timed firing signals to the cameras110 and 112 via the ADCs 308, to trigger camera exposure, according to astored flight plan and the real-time position and speed of the aircraft.If a camera 110 and/or 112 incorporates an auto-focus mechanism then thecomputer 300 also provides a focus signal to each such camera to triggerauto-focus prior to exposure.

The computer 300 fires the overview camera 112 and the detail cameras110 at the same rate. Alternatively, the computer 300 may fire theoverview camera 112 at a different rate to the detail cameras 110, i.e.either a higher rate or lower rate, to achieve a different overlapbetween successive overview photos, i.e. either a higher overlap or alower overlap, independent of the overlap between successive detailphotos. The computer 300 may fire the cameras simultaneously, or it maystagger the timing of the firing, e.g. to achieve a different alignmentof photos longitudinally, or to reduce peak power consumption.

The flight plan describes each flight line making up the survey, and thenominal camera firing rate along each flight line required to ensurethat the necessary overlap is maintained between successive shots. Thefiring rate is sensitive to the elevation of the terrain below theaircraft, i.e. the higher the terrain the higher the firing rate needsto be. It is adjusted by the computer 300 according to the actual groundspeed of the aircraft, which may vary from its nominal speed due to windand the pilot's operation of the aircraft.

The computer 300 also uses the flight plan and real-time GNSS positionto guide the pilot along each flight line via a pilot display 302.

As shown in FIG. 20, the position data from the GNSS receiver isoptionally augmented with orientation information from an inertialmeasurement unit 306 (IMU). This allows the computer 300 to provideenhanced feedback to the pilot on how closely the pilot is following theflight plan. In the absence of the IMU 306 the GNSS receiver connectsdirectly to the computer 300.

The computer stores the GNSS position (and optionally IMU orientation,if the IMU 306 is present) of each shot. This is used during subsequentprocessing of the photos to produce an accurate photomosaic.

Each camera 110 and 112 stores its shots locally, e.g. in removableflash memory. This eliminates the need for centralised storage in theHyperCamera system, and the need for a high-bandwidth data communicationchannel between the cameras and the centralised storage.

The GNSS position of each shot may be delivered to each camera 110 and112, to allow the camera to tag each photo with its GNSS position.

The cameras 110 and 112 are powered by a battery unit 320. The batteryunit 320 provides a voltage higher than the voltage required by allconnected components, e.g. between 24V and 28V, and the voltagerequirement of each connected component is provided via a DC-DCconverter 326. For example, a Nikon D800 camera requires less than 10V.Additional DC-DC converters 326 also provide appropriate voltages topower the computer 300, the pilot display 302, the GNSS receiver 304,and the IMU 306. For clarity these power connections are omitted in FIG.20.

The battery unit 320 contains two 12V or 14V batteries or a single 24Vor 28V battery. It contains a charging circuit that allows it to betrickle-charged from an aircraft with a suitable auxiliary power source322, allowing it to remain charged at all times. It may also be chargedon the ground from a ground power unit 324 (GPU).

The ADCs 308 and DC-DC converters 326 may be housed in a camera controlunit 310 (CCU). This may additionally include a USB interface to allowthe computer 300 to control the ADCs.

The DC-DC converters 326 that provide power to the cameras 110 and 112may be located in the CCU 310 or closer to the cameras in thedistribution boxes 150.

Photos captured by the HyperCamera are intended to be seamlesslystitched into a photomosaic, and FIG. 21 shows a photogrammetric processflow for efficiently creating a photomosaic from multi-resolutionHyperCamera photos. The process operates on detail photos 400 capturedby the detail cameras 110, and overview photos 402 captured by theoverview cameras 112.

The process consists of four main steps: (1) features are automaticallydetected in each of the photos 400 and 402 and matched between photos(step 410); bundle adjustment is used to iteratively refine initialestimates of the real-world three-dimensional position of each feature,as well as the camera pose (three-dimensional position and orientation)and camera calibration (focal length and radial distortion) associatedwith each photo (at step 412); each detail photo 400 is orthorectifiedaccording to its camera pose and terrain elevation data (at step 414);and the orthorectified photos (orthophotos) are blended to form thefinal photomosaic 404 (at step 416).

The accuracy of the photomosaic 404 derives from the high overlapbetween lower-resolution overview photos 402, while detail in thephotomosaic 404 derives from the higher-resolution detail photos 400.

As an alternative, as noted above, a survey may be flown with higheroverlap between the detail photos 400, and the photomosaic may becreated from the detail photos 400 only.

The photomosaic is typically stored as an image pyramid, i.e. withinwhich different (binary) zoom levels are pre-computed for fast access atany zoom level. Lower zoom levels in the pyramid are generated fromhigher zoom levels by low-pass filtering and subsampling, thus theentire pyramid may be generated from the detail-resolution photomosaic.As an alternative, lower zoom levels may be generated from a photomosaiccreated from the overview photos 402, in which case the overview photos402 are also orthorectified and blended as described above for thedetail photos 400.

An initial estimate of the camera pose of each photo, subsequentlyrefined by the bundle adjustment process (at step 412), is derived fromthe GNSS position of each photo, as well as its IMU-derived orientation,if available.

The terrain data used to orthorectify (at step 414) the detail photos400 may be based on 3D feature positions obtained from bundle adjustment(at step 412), or may be terrain data sourced from elsewhere (such asfrom a LiDAR aerial survey).

Automatically detected ground features may be augmented withmanually-identified ground points, each of which may have an accuratesurveyed real-world position (and is then referred to as a groundcontrol point).

The present invention has been described with reference to a number ofpreferred embodiments. It will be appreciated by someone of ordinaryskill in the art that a number of alternative embodiments of the presentinvention exist, and that the scope of the invention is only limited bythe attached claims.

The invention claimed is:
 1. A system for capturing aerial images, thesystem comprising at least one overview camera, a plurality of detailcameras, and a frame for holding the cameras, each detail camera havinga longer focal length than the at least one overview camera, each detailcamera mounted at a different angle laterally so that the fields of viewof the detail cameras overlap to form an extended lateral field of view,the frame attachable to the floor of an aircraft above a camera hole,thereby providing the cameras with a view of the ground below theaircraft through the camera hole.
 2. The system of claim 1, furthercomprising an adapter plate attachable to the floor of the aircraft, theframe attachable to the adapter plate.
 3. The system of claim 2, whereinthe adapter plate attaches to the floor by bolting to mounting pointsset into the floor.
 4. The system of claim 2, wherein the adapter plateattaches to the floor by bolting to seat tracks attached to the floor.5. The system of claim 1, wherein each detail camera is angled inwardstowards the center of the camera hole, thereby minimising the size ofthe camera hole required to accommodate the fields of view of the detailcameras.
 6. The system of claim 1, wherein the ratios of the focallengths of the detail cameras to the focal length of the at least oneoverview camera are between 4 and
 8. 7. The system of claim 1, whereinthe focal lengths of the detail cameras are between 85 mm and 800 mm. 8.The system of claim 1, wherein the focal lengths of the detail camerasare selected from the group comprising: 85 mm, 105 mm, 135 mm, 180 mm,200 mm, 300 mm, 400 mm, 500 mm, 600 mm, 700 mm, and 800 mm.
 9. Thesystem of claim 1, wherein the focal length of the at least one overviewcamera is between 10 mm and 200 mm.
 10. The system of claim 1, whereinthe focal length of the at least one overview camera is selected fromthe group comprising: 10.5 mm, 14 mm, 16 mm, 18 mm, 20 mm, 21 mm, 24 mm,28 mm, 35 mm, 40 mm, 45 mm, 50 mm, 55 mm, and 60 mm.
 11. The system ofclaim 1, wherein there are five detail cameras.
 12. The system of claim1, wherein the detail cameras have fixed-focus lenses focused atinfinity.
 13. The system of claim 1, wherein the detail cameras havevariable-focus lenses and auto-focus mechanisms.
 14. The system of claim1, wherein the at least one overview camera has a fixed-focus lensfocused at infinity.
 15. The system of claim 1, wherein the at least oneoverview camera has a variable-focus lens and an auto-focus mechanism.16. The system of claim 1, further comprising a computer systemconfigured to automatically fire the detail cameras during flight suchthat successive detail photos overlap longitudinally.
 17. The system ofclaim 16 further comprising at least one Global Navigation SatelliteSystem (GNSS) receiver, the computer configured to receive and storeposition data from the at least one GNSS receiver in real time.
 18. Thesystem of claim 17 further comprising an IMU, the computer configured toreceive and store orientation data from the IMU in real time.
 19. Thesystem of claim 16 further comprising a pilot display, the computerconfigured to provide flight instructions to the pilot via the cockpitdisplay.
 20. The system of claim 16 further comprising a stored flightplan, the computer configured to fire the cameras at a rate determinedfrom the flight plan, the real-time position of the aircraft, and thereal-time speed of the aircraft.